1. Field of the Invention
The present invention pertains to an image forming apparatus such as a copying machine or a laser printer, and more particularly to a technology to correct positional discrepancies pertaining to an image.
2. Description of the Related Art
In a color image forming apparatus, the image to be formed is generally broken down into the four colors of cyan (C), magenta (M), yellow (Y) and black (Bk), and after a toner image is formed for each color, these single-color images are transferred onto a recording medium such that each image overlays the preceding images, so as to form a multi-color image. Therefore, if the single-color images are not completely overlaid on top of the previous images, color discrepancies occur, resulting in poor image quality.
In particular, color discrepancies easily occur in a tandem-type image forming apparatus, and the formulation of a method to reduce this problem is a key task. A tandem-type image forming apparatus comprises image forming units that each form single-color images and are sequentially located along a transfer belt. The single-color images are overlaid onto the transfer medium in synchronization with the conveyance of the transfer medium while the image forming time is adjusted for each color.
In a tandem-type image forming apparatus, resist correction has conventionally been performed in order to correct the image forming positions and prevent color discrepancies. In resist correction, a resist mark that has a prescribed configuration is formed on the transfer belt by each image forming unit, and the resist marks are detected by means of an optical sensor to calculate the amounts of discrepancy in the positions of the single-color images.
When detecting the positions of the resist marks in such resist correction, however, in order to prevent color discrepancies and maintain image quality, very precise detection of several .mu.m to less than 20 .mu.m is required.
Conventionally, for the detection of the positions of the resist marks, an optical sensor that uses a CCD or a photodiode as the photoreceptor has been used. Available methods to confirm the position of the resist mark include (1) a method that seeks the center of the waveform from the leading and trailing edges of the waveform that is obtained when the line comprising the resist mark passes the sensor (sampling waveform), (2) a method that deems the point at which the maximum value is obtained to be the peak of the waveform, and (3) a method that seeks the centroid of the waveform from the sampling result.
Using these conventional detection methods, the shorter the resist mark sampling intervals are, the more reliably and precisely the resist mark can be detected. In this case, however, since the signals must be switched at high speed, the use of a CPU with high-level processing capabilities is required, which leads to an increase in cost. An additional problem is that the rapid switching of signals causes high-frequency noise, which adversely affects the resist mark detection signals and generates errors.
Where even minor errors occur in the detection signals, it becomes difficult to seek the accurate position of the resist mark and precise resist correction cannot be performed.